Well, in general I wonder if the team that wrote that part of Arsenal has ever seen the innards of any firearm.
Regarding an aftermarket integral suppressor, given a competent gunsmith, it really depends on the weapon. All you need is to have most of the barrel exposed. I can picture how you would do it for the modern AR-15/M16, which is probably the conceptual inspiration for the modular weapons of SR4.
For example, the M23 assault rifle from Arsenal. Remove the front sight and handgrip, rework the gas tube/piston (if any), and you can tap the barrel and assemble the baffle stack around it. More likely, the user would purchase a replacement barrel and integral suppressor as a single unit.
Most semiauto handguns could not take an integral suppressor, because the slide that surrounds the barrel is a vital part of the action. You would have to mout an extended barrel, then mount an integral suppressor on that, which defeats the entire purpose.
A revolver
would be an ideal candidate, since the entire barrel is exposed. Too bad about that cylinder gap.
QUOTE (Saint Sithney @ Oct 22 2009, 09:39 PM)

In SR, we're taking about an aftermarket add-on which doesn't increase the size of the gun since it doesn't change the concealibility modifier.
Can you imagine fitting an internal silencer into a Walther palm pistol? Or any CC pistol for that manner. Ridiculous.
Need to still have some pipe left after the gas vents out and when you're talking about maybe 2 inches of barrel, you've got no room left for rifling.
You always have room for rifling. When you picture gas vents in this context, think little pinholes, not the huge holes you might see at the end of a barrel for recoil management. Anyway, with a holdout, your ammo is probably subsonic already, so you don't need barrel vents. So, you end up with a 2" barrel sheathed in a 2.5" suppressor.